In 1973, Ya'alon was called up as a reservist during the Yom Kippur War. On October 15, 1973, his unit became the first IDF unit to cross the Suez Canal into Egypt. He continued fighting as part of the Israeli drive into the Egyptian mainland, and participated in the encirclement of the Egyptian Third Army. Following the war, he rejoined the IDF as a career soldier, and completed an officers' course. He was appointed a platoon commander, and then as a commander in officers' school. In 1978, he became a commander in the Paratroopers Brigade.[1]

During the 1982 Lebanon War, Ya'alon joined the Sayeret Matkal commando unit as a commander. He then rejoined the Paratroopers Brigade and was appointed commander of its 890th Battalion. During this time, he was wounded in the leg while leading a pursuit of Hezbollah fighters in Lebanon.[1]

Ya'alon took a sabbatical to the United Kingdom to study at the British Army's Camberley Staff College. Upon his return to Israel, he was promoted to the rank of Colonel and appointed commander of Sayeret Matkal. Ya'alon lead it to many notable achievements, for which the unit received four recommendations of honor. After his tenure as commander was finished, Ya'alon studied at the University of Haifa, obtaining a BA in Political Science, and took an Armored Corps course. In 1990, Ya'alon was appointed commander of the Paratroopers Brigade,[3] and two years later, became commander of the Judea and Samaria Division. On 10 December 1992, Ya'alon killed a militant from the Islamic Jihad Movement in Palestine with a hand grenade after the militant had shot dead a Yamam operative attempting to arrest him.[4] In 1993, he was appointed commander of an IDF training base, and commander of an armored division. In 1995, Ya'alon was promoted to Major General and appointed head of Military Intelligence. In 1998, he was appointed commanding officer of Israel's Central Command. He was serving in this position when the Second Intifada was launched in September 2000.

In February 2005, Defense Minister Shaul Mofaz decided not to prolong Ya'alon's service as Chief of Staff for another year. This marked the climax of tensions between Mofaz and Ya'alon, which had arisen partly through Ya'alon's objection to the Gaza disengagement plan. On 1 June 2005, Ya'alon retired from the army, and Dan Halutz, his successor as Chief of Staff, oversaw the disengagement.

As Defense Minister Ya'alon decided to continue to manufacture and purchase Merkava tanks for the IDF, after the whole project was in question due to budget issues and the overall discussion of the necessity of tanks in modern battlefields. During Ya'alon's tenure, foreign sources claimed that the Israeli Air Force launched several airstrikes on advanced weapons deposits in Syria before they were transferred to Hezbollah.

In January 2015, the leader of Lehava anti-assimilation group, Ben-Zion Gopstein, voiced harsh criticism against Ya'alon after Channel 2 reported that he was expected to categorize Lehava as a terrorist organization. Ya'alon ordered the Shin Bet and the Defense Ministry to assemble evidence required for the classification. Lehava promotes the ideology of the late Jewish Defense League leader Meir Kahane. Three members of Lehava were arrested and indicted in 2014 for committing arson and spray-painting anti-Arab graffiti at the Max Rayne Hand in Hand (Yad B’Yad) Bilingual School in Jerusalem and Lehava's leader Ben Zion Gopstein along with other group members were arrested shortly thereafter for incitement.[13] The arson incident received international attention.[14] Gopstein said "I suggest that [Ya’alon] aim to outlaw the Islamist Movement and then preoccupy itself with an anti-assimilation group... Instead of taking care of an enemy of Israel, the defense minister is trying to win over votes from the Left [by] taking on Lehava. The group acts to save the daughters of Israel [Jewish women] and deserves the Israel Prize."[15]

In February 2015 Ya'alon took a political stand on gay marriage. In a speech he promised to help promote LGBT rights in Israel, and in particular noted he will support the establishment of civil marriage laws in Israel that would allow same-sex couples to be recognized as a family unit.[16]

On 27 August 2002, he told the Haaretz newspaper: "The Palestinian threat harbors cancer-like attributes that have to be severed. There are all kinds of solutions to cancer. Some say it's necessary to amputate organs but at the moment I am applying chemotherapy."[17] In January 2004, he publicly stated that the thirteen Sayeret Matkal soldiers who refused to serve in the Israeli-occupied territories were taking the unit's name in vain.

In January 2008, during a discussion at IDC Herzliya, Ya'alon said "There is no way to stabilize the situation all over the world and especially in the Middle East without confronting Iran."[18] According to The Sydney Morning Herald Ya'alon said: "We have to confront the Iranian revolution immediately. There is no way to stabilize the Middle East today without defeating the Iranian [government]. The Iranian nuclear program must be stopped."

When asked whether "all options" included a military deposition of Ahmadinejad and the rest of Iran's current leadership, Ya'alon told The Herald: "We have to consider killing him. All options must be considered."[19]

In August 2009, Ya'alon visited the ruins of Homesh,[20] a settlement that was destroyed in Israeli disengagement from Gaza in 2005 and toured Israeli settlements in the north of the West Bank, considered as un-authorized outposts. He said that these communities are all legitimate and should not be called "illegal."[21] In addition, he participated in a convention of Manhigut Yehudit ("Jewish Leadership"), the more right-wing Settlers' segment within the Likud right-wing Party, in which he condemned the disengagement plan, called Peace Now a "virus"[22][23] and said that "We become accustomed to Arabs being permitted to live everywhere, in the Negev, Galilee, Nablus, Jenin, and [on the other hand] there are areas where Jews are not allowed to live. We caused this." He also stated that, "regarding the issue of the settlements, in my opinion Jews can and should live everywhere in the Land of Israel. Now, ... first of all, every settlement needs to get the approval of the authorities, and what goes up on the spot, in contradiction to these decisions and so on is not legitimate. It's against the law".[23]

Later, after meeting with PM Netanyahu, Ya'alon retracted parts of his statements and said that he "recognized the importance of democratic discourse and respecting other opinions."[24] Ya'alon explained that, indeed, all Israelis want peace, now. He stressed, however, the need to accept the fact that peace will not come immediately, otherwise it "hurts Israel." Ya'alon stated that, in his view, the way of thinking that Israel just needs to give one more piece of land and then it will have peace is a kind of "virus."[25]

U.S. Secretary of State John Kerry and U.S. engagement in the peace process[edit]

In January 2014, Ya'alon was quoted in an Israeli newspaper calling U.S. Secretary of State John Kerry "obsessive" and "messianic," and claiming that the "only thing that can save us is if Kerry wins the Nobel Prize and leaves us alone."[26][27] The remarks attributed to him precipitated a diplomatic row with the U.S. State Department and elicited criticism of members of Israel's government. In October 2014 during a visit to the U.S. Ya'alon met with his counterpart, Defense Secretary Chuck Hagel, but was denied requests to meet with Vice President Joe Biden and Secretary of State John Kerry.[28] A U.S. official told the Jerusalem Post that the snub should come as no surprise.[29]

In October 2014, Ya'alon supported a plan that would effectively ban Arab workers from buses used by Jewish settlers. Ya'alon said that his plan was based on security needs. The IDF said that there was no security issue with integrated buses given that all Judea and Samaria Arab workers go through security checkpoints.[30] When Ya'alon's move was criticized as being a form of apartheid, Yehuda Weinstein, Israel's Attorney General, demanded that Ya'alon explain his decision.[31]

In late 2014 at a meeting with Jewish settlers in Judea and Samaria who questioned Israel's apparent delay in settler construction, Ya'alon said that settlement construction represented the greatest growth in Israel and with regard to any apparent slowdown, he declared, "It's temporary. There is a certain administration now in the United States, which is leading this policy. This administration will not stay forever and I hope it's temporary." He commented that it is important for Israel not to antagonize the U.S. at the moment, because American support is needed in countering diplomatic undertakings at the U.N. that he claimed were "anti-Israel." “The publication of construction plans, even in Jerusalem, draws a response from various bodies, including our friends, that attack us,” he said. But he noted that he very much wanted to approve construction plans.[32] Ya'alon's remarks about not antagonizing the U.S. came at a time when an opinion poll showed that the "overwhelming majority" of Israelis believe their country's relationship with the U.S. has been hurt as a result of the poor relationship between Obama and Netanyahu.[33][34] In response to the Ya'alon's remarks, which were broadcast of Israeli Army Radio, a spokesperson for Ya'alon claimed, "this does not diminish in any way the appreciation and esteem Minister Ya'alon has for the contribution of the U.S. and its government to the security of Israel and the fact that the U.S. is Israel's best friend and strategic anchor."[35] Responding to the latest of Ya'alon negative comments about the Obama administration, U.S. State Department spokeswoman Jen Psaki said, "This administration’s opposition to settlements is fully consistent with the policies of administrations for decades, including of both parties. So the notion that that would change is not borne out by history."[36]